The studies performed in the second year of our project have demonstrated that many neglected or ignored technical manipulations in the course of bone marrow transplantation (BMT) profoundly affect the permanent engraftment of allogeneic BM and induction of chimerism across the H-2 barrier in mice. As documented in our reports, sex-matching of donor-recipient, timing of BM inoculation after irradiation, maintenance of an intact cell population (including cytotoxic T cells) in the donor marrow and removal or maintenance of BM internal factors all play a major role and must be combined and adjusted independently from the genetic diversity of the partners. The striking differences observed when one or those parameters has been disregarded point to the prominent role of hematological and endocrinological factors in BMT. The application of our system and its many technical improvements has allowed us to obtain permanent chimerism and prevention and/or therapy of leukemia in leukemia-prone, AKR mice transplanted with BM from leukemia-resistant C57BL mice. The BM factors differ depending on the H-2 character. This shows the individual endocrine character of the BM. In the case of AKR (H-2k) mice, chimerism is achieved only when the donor marrow (H-2b) is transferred together with its own BM-derived microenvironmental extracellular factors. A further analysis of the heterologous (rabbit or calf) marrow-regulating factors (MRF) applied has shown that a heat-resistant component is contained that promotes engraftment of allogeneic BM. We are presently searching its chemical character. Its chemical identification is in progress and is our main task for the next year, combined with studies on its mechanism of action. The model is now available for purification and production of MRF in view of its possible therapeutical application. In fact, use of a chimerism-inducing physiological substance in clinical BMT would eliminate many if not all of the problems occurring when a deficient engraftment in HLA-identical or different partners results in emergence of acute or chronic GVHD and its often fatal outcome. (TA)